The water contact angle measurements Figure 3 (b) confirm that the natural nutshells (NNSs) are hydrophilic in nature regardless of any treatments. This indicates that water can easily enter their micro and nanochannels which is essential to the generation of streaming potential or streaming current. Figure 3 (b) also demonstrates the enhanced hydrophilicity of the nutshells after being treated with acetone and ethanol solutions and subsequently dried in a nitrogen environment. This treatment process reduces the nonproductive adsorption between lignin and cellulose proteins, and hence the hemicellulose and lignin components are partially weakened and removed.^[42] Furthermore, the execution of plasma treatment caused an incredibly hydrophilic surface which is confirmed by the water contact angle measurement (S-V1).

Therefore, more hydrophilic functional groups are exposed. Concurrently, densities showed small declines (Figure-S 2 ), means this treatment slightly enhanced porosity. It should be noted that this treatment didn’t remove the lignin contents confirmed by the FT-IR spectra Figure 3 (a) and CLSM images Figure 3 (d). To enhance the surface areas, additional treatments are required to remove the lignin and break the lignin and cellulose links. However, the elimination of lignin will cause a structural collapse that will compromise the stability of the device.^[43] Thus, the minor enhancement of the surface areas and porosity by these treatments ensures the long-term stability of the device. Out of these four NSs examined, the Almond Shell (AS) has the highest surface areas and porosity, while the pecan shell (PS) has the lowest surface areas and porosity, although the variations are quite small. Walnut shell (WS), on the other hand, demonstrates a well-balanced mix of surface areas, porosity, zeta potentials, and superior mechanical stability.^[44] Therefore, this study focuses on the functionally graded WS structure for most analyses.

Figure 3 (c) shows how color perception changes while exposed to the different treatments of WS. After being freeze-dried for 12 h, the color of the WS samples transforms from a dark orange tone to a pale orange.